Japanese Unexamined Patent Publication No. 4-365920 discloses a device for purifying an exhaust gas of an internal combustion engine having a plurality of cylinders divided into first and second cylinder groups, in which the device includes an NH.sub.3 synthesizing catalyst arranged in a first exhaust passage which is connected to the first cylinder group, the NH.sub.3 synthesizing catalyst synthesizing ammonia NH.sub.3 from nitrogen oxide NO.sub.X in the inflowing exhaust gas when the exhaust gas air-fuel ratio of the inflowing exhaust gas is rich, and an interconnecting exhaust passage interconnecting the first passage downstream of the NH.sub.3 synthesizing catalyst and a second exhaust passage which is connected to the second cylinder group, wherein the first cylinder group performs a rich operation to supply NH.sub.3 to the interconnecting passage to thereby reduce NO.sub.X in the exhaust gas exhausted from the second cylinder group which performs a lean operation, using the NH.sub.3. In the device, while the rate of fuel consumption of the engine is made as low as possible by increasing the number of cylinders composing the second cylinder group, NO.sub.X produced by the second cylinder group is purified by NH.sub.3 so that the amount of NO.sub.X discharged into the ambient air is reduced as much as possible.
The NH.sub.3 synthesizing catalyst described above may be comprised of a three-way catalyst which includes a precious metal such as palladium Pd, platinum Pt, or rhodium Rh. However, when the NH.sub.3 synthesizing catalyst is composed of the above mentioned three-way catalyst, if the exhaust gas air-fuel ratio of the exhaust gas flowing to the NH.sub.3 synthesizing catalyst is kept rich for a long time, i.e., if the NH.sub.3 synthesizing catalyst is kept contact with the exhaust gas of which the exhaust gas air-fuel ratio is rich, the catalytic activity of the NH.sub.3 synthesizing catalyst is relatively easily lost. As a result, NH.sub.3 cannot be synthesized excellently, and thus NO.sub.X cannot be purified excellently.